Echo sounding technique

ABSTRACT

An ultrasonic echo sounding technique for reducing echoes from reflectors outside the field of interest comprising: a substantially annular ultrasonic transducer assembly having a plurality of equally sized segmental transducer elements whose radiation surfaces are inclined towards the annular axis; a transmitter with a generator; phase shifting means for driving selected transducer elements with signals having relative phase shift; transmitter switches for optionally switching the aforementioned apparatus between generator and transducer to produce first and second waves; a receiver with a first and second adder; phase rotation means connected with said second adder; receiver switches operated in synchronism with the transmitter switches for optionally switching the transducer elements between the phase rotation means and first adder; rectifier means for rectifying the outputs of the adders; a delay line for delaying signals from the first adder; and, subtraction means for taking the difference of the first and second adder outputs and displaying the results.

United States Patent Burckhardt et a].

[ Sept. 17, 1974 ECHO SOUNDING TECHNIQUE Primary Examiner-Richard A. Farley [75] Inventors: Christoph Benedikt Burckhardt, 'i Agent or Ezrm samuel Jon Muttenz; Pierre Andre Saxe Mark Hopkms Grandchamp, Arlesheim, both of Switzerland; Heinz Hoffman, [57] ABSTRACT Grenzach, Germany An ultrasonic echo sounding technique for reducing [73] Assi nee Hoffmanlda Roche Inc Nutle echoes from reflectors outside the field of interest g N J comprising: a substantially annular ultrasonic transducer assembly having a plurality of equally sized seg- [22] Filed: Feb. 7, 1973 mental transducer elements whose radiation surfaces are inclined towards the annular axis; a transmitter [21] Appl 330364 with a generator; phase shifting means for driving selected transducer elements with signals having relative [30] Foreign Application Priority Data phase shift; transmitter switches for optionally switch- Feb. 11, 1972 France 72.2032 ing the aforementioned apparatus between generator Dec. 7, 1972 France 72.17818 and transducer to Produce first and second waves; a receiver with a first and second adder; phase rotation [52] US. Cl 340/1 R means Connected with said second e cei 51 Int. Cl. G015 9/66, GOls 7/66 SWiteheS Operated in synehronism with the transmitter [58] Field 61 Search 340/1 R, 3 R, 5 R, 10, switches for Optionally Switching the transducer 340 5 ments between the phase rotation means and first adder; rectifier means for rectifying the outputs of the 5 References Cited adders; a delay line for delaying signals from the first UNITED STATES PATENTS adder; and, subtraction means for taking the differ- 3 086 195 4/1963 H d 340/15 ence of the first and second adder outputs and displaya 1 ay 3,341,807 9/1967 Lobdell 340 1 R mg the msults' 16 Claims, 7 Drawing Figures 42 48 57 2 s t r -6O 13- 58 3 9 |-l r :1 e2

-l2O 51 32- ADDE 7 4 IO ll Y .J 3 5| -l80 9 620 45 5 I r fDELAY ADDER \LINE PAIENIE SEP 1 7 m4 SHED 1 BF 4.

PATENIEDSEPI 1 3. 836.948

SHEET l 0F 4 cos t 75 w 86 76 g f' -g FILTER 5 SUB [5 3 x w q 62 78 MULTI- FILTER sin wf-- 8| FIG. 6 cos wt 5 sin an 5 859 62 "-F|| TER 9o PLIER j$ MULTI FILTER (DELAY PLIER LINE FIG. 7

ECHO SOUNDING TECHNIQUE BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a method for reducing the echoes from reflectors outside the focal line in an echo sounding method using a convergent ultrasonic field with a substantially annular cross-section and apparatus for performing this method.

2. Description of the Prior Art The prior art already discloses a method which permits good focusing of the ultrasonic field over a large depth. However, such focusing suffers from the disadvantage of relatively large lateral maxima, that is to say undesirable echoes of reflectors positioned to the side of the axis, which may have a relatively large amplitude. It is the object of the present invention to eliminate this disadvantage by reducing the lateral maxima.

SUMMARY According to the invention this is achieved by: transmitting a first pulse of ultrasound to generate a first ultrasonic wave having a substantially annular crosssection in phase over the entire cross-section; receiving echoes of the first ultrasonic wave from reflectors in the focal line and outside thereof; transmitting at least a second ultrasonic wave with one further pulse of ultrasound having a substantially annular cross-section; by receiving echoes from reflectors in the focal line and outside thereof; every further ultrasonic wave being not in phase over the entire cross-section and/or the echoes being provided with a phase shift which varies over the cross-section; by providing the electrical signals obtained from the echoes with weighing coefficients and by adding echoes having a substantially identical travel time.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows an annular ultrasonic transducer comprising 12 sectors.

FIG. 2 is a block circuit diagram ofa transmitting circuit.

FIG. 3 is a block circuit diagram of a receiving circuit.

FIG. 4 is a graphical view of signal amplitudes.

FIG. 5 is a diagrammatic view of a further embodiment of the invention.

FIG. 6 is a block circuit diagram ofa modified receiving circuit.

FIG. 7 is a block circuit diagram ofa further modified receiving circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENT The annular ultrasonic transducer assembly 20, illustrated in FIG. 1, has an annular casing 21 of substantially U-shaped cross-section. the axis of symmetry of the cross-section being inclined relative to the annulus axis and intersecting same at a specific distance. The open side of the U-shaped cross-section faces the intersecting point and therefore the annulus axis. The aperture formed by the casing 21 is provided with 12 equally sized segmental transducer elements 1-12 which together define a ring. These transducer elements each comprise a piezoelectric layer on a damping block of epoxy resin/tungsten. The transducer elements are acoustically insulated from the casing 21 and from each other by means of cork plates 22. The transducer elements are electrically connected by means of a metallic layer on their front faces applied by vacuum deposition connected to the casing and in addition by individual connecting leads (not shown) which extend through bores in the casing and in the damping blocks to the rear faces of the piezoelectric layers. A coating 23 of epoxy resin matches the sound impedance to water and provides mechanical protection of the piezoelectric units.

In operation, each piezoelectric unit transmits an ultrasonic beam. The ultrasonic field produced in this manner is focused along a focal line segment at a specific distance from the ultrasonic transducer on the annulus axis. The length of the focal line segment defines the depth over which good lateral resolution is achieved. The distance between the focal line segment and the ultrasonic transducer is defined substantially by the angle of inclination of the piezoelectric oscillators with respect to the annulus axis. The length of the focal line segment is defined principally by the width of the piezoelectric oscillators. The shape of the ultrasonic field transmitted by the piezoelectric oscillators may be characterized by the cross-section formed by intersection with any plane which is perpendicular to the annulus axis and disposed between the transducer and the focal line segment and referred to hereinafter as crosssection of the ultrasonic field. In the present case. the cross-section is substantially annular with its diameter decreasing with an increasing distance from the transducer to the focal line segment.

In the block circuit diagram of the transmitting circuit illustrated in FIG. 2, the transducer elements 1-12 are symbolically indicated by their numbers in blocks. The transducer elements are driven by a transmitter 25 via amplifiers 26-31 and switches 32-37, all the switching arms being gauged together to either be in position I or in position ll. defining two different connecting paths from the transmitter depending on the position of the switches. The first path comprises the conductor 38 connecting transmitter 25 to the contact I of the switches 32-37 to drive all transducer elements l-l2 in phase via the amplifiers 26-31. The second path contains a delay line 39 with a plurality of tappings connected to the contacts 11 of the switches 32-37. The tappings are selected so that each individual time delay corresponds to a transmitting frequency phase difference of 60. With the switch in position 11, in relation to the ultrasonic waves transmitted by the transducer elements 1 and 7, the transducer elements 2 and 8 will transmit ultrasonic waves which are phase shifted by +60, the transducer elements 3 and 9 by +l20, the transducer elements 4 and 10 by +1 the transducer elements 5 and 11 by +240 and the transducer elements 6 and 12 by +300.

The transducer elements l-l2 simultaneously function for receiving the reflected ultrasonic waves. As depicted in FIG. 3, they are connected via amplifiers 41-46 to switches 47-52 in the same grouping as for transmitting. The switches 47-52 may all be either in position I or in position 11. They also all connect into the appropriate switch positions I or 11 together with the switches 32-37 of the transmitting circuit. The contacts I of the switches 47-52 are connected to an adder unit the output of which is connected via a rectifier circuit 53a to a delay line 54. The delay line 54 is dimensioned so that its delay time corresponds to the time interval between two successive transmitted pulses. The output of the delay line 54 is connected to a first input of subtraction unit 55 which output is connected to a cathode ray oscilloscope 56. The contacts 11 of the switches 47-52 are connected to phase rotation elements 57-61 whose outputs extend to an adder unit 62. The contact 11 of the switch 47 is directly connected to one input of the adder unit 62. The phase rotation elements 57-61 impart a phase shift of the same magnitude as in the transmitting circuit but with a reversed sign to the pairs of received signals from the transducer elements. The output of adder unit 62 is connected via a rectifier circuit 620 to a second input of the subtraction unit 55.

The method of operation of the system comprising the annular ultrasonic transducer. the transmitting circuit and the receiving circuit is as follows: initially all switches. including those of the transmitting circuit as well as those of the receiving circuit, are in position I. A signal generated by the transmitter 25, passes via the conductor 38, the switches 32-37 and the amplifiers 26-31 to the transducer elements 1-12 which are all energized in phase. The transducer elements transmit ultrasonic waves whose cross-sections substantially represent ring segments. The entire ultrasonic field. comprising beams which are transmitted by the individual transducer elements, therefore has a substantially annular cross-section. All ultrasonic beams radiated by the individual transducers are in phase and accordingly. the entire ultrasonic field is in phase. Reflected ultrasonic waves are received by the transducer elements 1-12, are amplified in the amplifiers 41-46 and are supplied via the switches 47-52 to the addition network 53. All signals are added in the adder unit 53, are rectified by the circuit 53:! and are fed into the delay line 54.

All switches 32-37 and 47-52 are then shifted into position ll. The transmitter now delivers a second pulse which, by means of the tapped delay line 39, is developed into six signals having the previously described relative phase shift. The transducer elements 1-12 are driven in pairs by means of the aforementioned signals. The ultrasonic transducer therefore radiates an ultrasonic field whose cross-section comprises a plurality of sectors with relative phase shift between them.

Reflections of the second ultrasonic pulse are received by the transducer elements 1-12, are converted into electric signals and are amplified by the amplifiers 41-46. The output signal ofthe amplifier 41 is supplied directly to the adder unit 62. The output signals of the amplifiers 42-46 are supplied to the phase rotation elements 57-61 where they are subjected to a phase shift of the same magnitude as that applied in the transmitting circuit but having a negative sign. The phaseshifted output signals of the phase rotation elements 57-61 are also supplied to the adder unit 62. At the adder unit 62 the supplied signals are added. The output signal of the adder unit 62 is rectified and supplied to the subtraction network 55. The received signal of reflections of the first ultrasonic pulse due to delay line 54, arrives at the subtraction network 55 simultaneously with the adder unit 62 output signal. The two signals are then subtracted from each other and the result is displayed on the cathod ray oscilloscope. 1n the resultant signal the amplitudes of the echoes derived from reflectors in the zone of the lateral maxima are substantially smaller than those obtained from a simple process comprising only a single step.

In the previously described embodiment the phase shift selected for each transducer element is always proportional to the (15,, of the transducer element on the ring. The angle 4),, of the n'" transducer element is defined for example as the angle between the center of the n transducer element and the center of the first transducer element. the center of the first transducer element being arbitrarily defined as 42,, O.

The method in which the phase shift varies in steps from one transducer element to the next for the second or the further pulses represents an approximation of the general principle in which the phase varies continuously over the annular ultrasonic transducer, that is to say proportionally to the angle (1) of an imaginary infinitesimally small transducer element.

The principle of the method according to the invention may be described in general terms on the basis of the simplified theoretical considerations set out below. The second or the further pulses are transmitted by an annular ultrasonic transducer whose (infinitesimally small) elements therefore oscillate with the phase where (1) refers to the angle of one element on the ring. The resultant ultrasonic field may be defined by its cross-section. This has a phase which varies in proportion with the angle d). The sound amplitude a(r) in the zone of the focal line in this system is defined by the following expression:

a(r) J (211' r r/AR) e In this expression J refers to the Bessel function of the second order, A to the wavelength ofthe ultrasonics vibration in the particular medium, r, to the radius of the annular transducer. r to the radial co-ordinate of amplitude distribution as measured from the axis, R refers to the distance from the ring, and e represents the phase factor.

The signals of the received echoes are then multiplied by the phase factor e 'wherein 4) once again is the angle of the element in question. The amplitude S (r) of the received echo will then be expressed by (3) if the reflectors are irradiated with an ultrasonic field in accordance with equation (2). The expression S (r) is zero on the axis, that is to say it supplies only the echoes from reflectors in the zone of the lateral maxima.

The amplitude of the echo signal of the first pulse which is transmitted and received without phase shift is expressed by If equation (3) is subtracted. i.e., the echoes of reflectors in the lateral maxima. from equation (4) i.e.,

from the echoes of reflectors of the axis plus the echoes of reflectors in the lateral maxima. this will provide 51010) S (r) S (r) =1 (ZTTIAF/AR) J (21rr r/AR) The amplitude S,,,,(r) of the resultant signal is shown in FIG. 4 together with the value S ,(r). It will be seen that the lateral maxima of S,,,,(r) diminish substantially more rapidly than the lateral maxima of S.,(r).

As may be seen by reference to FIG. 4, the spot answer thus obtained still has certain lateral maxima. The spot answer may be improved by transmitting additional pulses in which each element oscillates in phase k (b, multiplying with the phase factor e" on reception, multiplying the signals thus obtained by a factor C and adding or subtracting them, whereby it is an integer. The total signal will then be of the form (6) where 1,, is the Bessel function of k"' order.

ALTERNATIVE EMBODIMENTS In addition to the embodiment described hereinabove, in which the phase shift of the individual beams of the ultrasonic field is obtained electronically, the method according to the invention may also be performed by apparatus in which transmitting and receiving ofthe ultrasonic pulse without phase shift, transmitting of the ultrasonic pulse in which the individual beams of the sonic field have relative phase shift, and receiving of the reflections ofthese ultrasonic pulses in which the individual beams of the ultrasonic field have relative phase shift, are performed with three different independent annular ultrasonic transducers. This embodiment involves reduced circuit complexity but calls for greater complexity for the manufacture of the assembly of the ultrasonic transducer or transducers respectively. Apparatus of this kind is shown diagrammatically in FIG. 5. Three annular ultrasonic transducers 63, 64 and 65 are provided and are arranged concentrically relative to each other. The middle transducer 63 is provided to transmit and receive ultrasonic waves without phase shift and is connected via contact I of a switch 66 to a transmitter 67 and via contact I of a switch 68 to an amplifier 69. The output of the amplifier 69 is connected to a rectifier circuit 69a which in turn is connected to a switch 70 whose contact I is connected to a delay line 71. The output of said delay line 71 extends to a subtraction network 72 whose output is connected to a cathode ray oscilloscope 73.

The transducer 64 transmits an ultrasonic wave having an annular cross-sectional surface and having a phase which varies in proportion to the angle (1). To this end the transducer is mounted on a portion which helically projects in the direction of propagation of the transmitted waves relative to the plane of the middle transducer whereby the pitch is selected so as to produce the desired phase shift. The transducer 64 is connected to the contact II of the switch 66.

The transducer 65 receives reflections of the ultrasonic field transmitted by the transducer 64. The piezo electric oscillator is mounted on a support which is helically recessed relative to the plane of the middle transducer and the direction of propagation of the transmitted waves, the pitch of which is the same as that of the transmitting transducer 64 since the latter is to provide the received signals with phase shift of the same magnitude as that of the transmitted signal but with a reversed sign. The transducer 65 is connected to the contact ll of the switch 68. The switch has a contact II which extends directly to one input of the subtraction unit 72.

In operation, all switches 66, 68 and 70 are in position I for a first pulse. A first pulse passes from the transmitter 67 to the ultrasonic transducer 63 thus causing the latter to emit an ultrasonic wave which is in phase over the entire cross-section. Echoes reflected by a reflector are once again received by the transducer 63 and are supplied via the switch 68 to the amplifier 69 and via the rectifier circuit 69a and the switch 70 to the delay line 71. All switches are subsequently changed into position II. A second pulse of the transmitter 67 passes to the transducer 64 which emits an ultrasonic wave whose phase in the cross-section is pro portional to the angle (b. The ultrasonic waves which are reflected by a reflector are received by ultrasonic transducer 65 and are provided with a phase shift which is opposite to that applied during transmitting. The electric signals delivered by transducer 65 are supplied via switch 68, amplifier 69, rectifier circuit 69a and switch 70 to the subtraction unit 72. The delayed received signal from the first pulse arrives simultaneously with the previously mentioned signal in the subtraction unit 72. The two signals are subtracted from each other in this network and the result is displayed on the cathode ray oscilloscope. The result obtained is the same as in the previously described first embodiment.

Another possibility of obtaining an angle dependent phase shift in the transmitting transducer 64 and in the receiving transducer 65 is to mount rings in front of the transducer, which consist of a suitable material for example acrylic resin and the thickness of which varies as a function of the angle (b.

In another solution to the problem, in which the phase varies in steps, the transducers comprise individual sectors which are offset relative to each other in steps with respect to depth. In another modification the transducers are subdivided into sectors and phase rotation elements are permanently incorporated into the electric leads.

The receiving circuit described by reference to FIG. 3 represents a simplified embodiment in the sense that it contains simple rectifier circuits for rectifying the echo signals. This simplified circuit provides useful results in the investigation of simple objects. To investigate complicated structures it is frequently advantageous to provide phase-corrected rectification in place of simple rectification. A receiving circuit, correspondingly modified for the l2-part ultrasonic transducer, is illustrated in FIG. 6. The output of the adder unit 62 is connected to one input of each of the two multiplier units 74 and 77 whose second inputs are supplied with the signals cos 0) r and sin to t. The output of the multiplier unit 74 is connected via a low-pass filter 75 to one input of a subtraction unit 76, the output of the multiplier unit 77 being connected via a low-pass filter 78 to a subtraction unit 79. The two low-pass filters eliminate the frequency portion close to the transmitted frequency. The output of the adder unit 53 is connected in corresponding manner to the inputs of two multiplier units 80 and 83 whose second inputs are once again supplied with the signals cos 1 r and sin 1 t. The output of the multiplier unit 80 is connected via a low-pass filter 81 and a delay line 82 to the second input of the subtraction unit 76 while the output of the multiplier unit 83 is connected via a low-pass filter 84 and a delay line 85 to the second input of the subtraction unit 79. The outputs of the two subtraction units 76 and 79 are connected to inputs of a computer circuit which forms the root from the square of the two subtracted signals. The output of the computer circuit 86 is connected to the cathode ray oscilloscope 56.

FIG. 7 illustrates another embodiment of a receiving circuit with phase-corrected rectification for the 12- part ultrasonic transducer. The output of the adder unit 62 is connected to a multiplier unit 88 the second input of which may be optionally supplied via a switch 87 with a signal source of cos 1 r or sin cu r respectively. The output of the multiplier unit 88 is connected via a low-pass filter to one input ofa subtraction unit 90. The output of the adder unit 53 is correspondingly connected to one input of a multiplier 92 whose second input may be optionally supplied via a switch 91 with a signal source of cos (n t or sin (1) I. The output of the multiplier 92 is connected via a low-pass filter 93 and a delay line 94 to the second input of the subtraction unit 90. The two switches 87 and 9] of this embodiment remain in position A for the duration of two pulses and then in the second position B for the duration of two further pulses. The output of the subtraction unit 90 is connected via a rectifier circuit 95 to the cathode ray oscilloscope 56. The sum of the absolute values of the two added signals obtained from the first and second pulse pair and being approximately equal to the root of the sum ofthe squares, is displayed on the cathode ray oscilloscope 56.

We claim:

1. ln an ultrasonic echo sounding technique a method for reducing echoes from reflectors outside a focal line with ultrasonic waves having substantially identical travel times comprising: generating a first convergent ultrasonic wave of substantially annular cross-section in phase over the entire cross-section; deriving from reflected ultrasonic echoes from the first wave a first set of electrical signals; generating a second convergent ultrasonic wave of substantially annular crosssection out of phase over the entire cross-section; deriving from reflected ultrasonic echoes from the second wave a second set of electrical signals; modifying the second set of electrical signals with phase correcting coefficients; and. comparing said first and second sets of electrical signals to provide a resultant signal indicative of the reflector at the focal line.

2. A method according to claim 1 including providing each second wave generated with a predetermined phase shift pattern over the entire cross-section. and providing said phase correcting coefficients with values identical to that applied during the generating step but having a reversed sign.

3. A method according to claim 1 including storing the derived first set of signals until said second set of signals are derived.

4. A method according to claim 1 where the phase of each successive generated ultrasonic second wave is uniformly proportioned at each point defined by angle (1: of their respective cross-sections.

5. A method according to claim 4 whereby the phase is It (1:, It being an integer.

6. A method according to claim 5 whereby k 2.

7. A method according to claim 4 whereby 5 assumes a number of discrete values so that the phase of each in the ultrasonic second waves varies in steps over the cross-section.

8. A method according to claim 7 whereby integral multiples of the discrete values of d) are equal to 2 1r.

9. A method according to claim 7 whereby the stepped change is obtained by phase shift in each the generated ultrasonic second waves.

10. Ultrasonic echo sounding apparatus for reducing echoes from reflectors outside a focal line with ultrasonic waves having substantially identical travel times comprising: annular assembly means having a plurality of identically sized segmental ultrasonic transducer means whose radiation surfaces are inclined towards an axis defined by the annular assembly means; generator means for driving said transducers; phase shift means adapted for connection between said generator means and transducer means; receiver means including a first and second adding means; said second adding means including phase rotation delay means; switch means operative in a first position for connecting said transducer means with said generator means and said first adding means, and in a second position for connecting said transducer means with said generator means through said phase shift means and said second adding means; and. means for comparing the output of said delay means and second adding means.

11. Apparatus according to claim 10 wherein the phase shift means comprises a delay line which is tapped at regular intervals for connection to preselected ones of said transducer means.

12. Apparatus according to claim 10 including: rectifier means connected from each of said adding means; and indicating means connected from said comparison means.

13. Apparatus according to claim 10 whereby said ultrasonic transducer means includes twelve transducer elements commonly connected in groups of two through the switch means for being driven and delivering signals to the receiver means.

14. Apparatus according to claim 13 whereby the outputs of the phase shift means have a relative phase difference of 60 and the phase rotation means apply the same phase shift with reversed sign.

15. Apparatus according to claim 11 whereby the rectifier means includes first and second multiplier means for respectively multiplying their output signals by cos (a r and sin to I; first and second low pass filters respectively connected from said multiplier means for eliminating the frequencies close to the transmission frequency; and. circuit means for forming the root from the sum of the squares of their imputs.

l6. Ultrasonic echo sounding apparatus for reducing echoes from reflectors outside a focal line with ultrasonic waves having substantially identical travel times comprising: transmitter means; first, second and third annular ultrasonic transducer with annular radiation surfaces means having a common annulus axis; said first transducer means having a radiation surface inclined relative to said axis with edges disposed substantially in a plane perpendicular to the axis; said second transducer means having a radiation surface with edges disposed on a helical surface forming a raised portion with respect to said plane; said third transducer means having a radiation surface with edges disposed on a helical surface representing an indentation with respect to said plane; signal processing means; delay line means;

means, and connecting in a second position the second transducer means to the transmitter means and the third transducer means directly to the subtraction means via said processing means. 

1. In an ultrasonic echo sounding technique a method for reducing echoes from reflectors outside a focal line with ultrasonic waves having substantially identical travel times comprising: generating a first convergent ultrasonic wave of substantially annular cross-section in phase over the entire cross-section; deriving from reflected ultrasonic echoes from the first wave a first set of electrical signals; generating a second convergent ultrasonic wave of substantially annular cross-section out of phase over the entire cross-section; deriving from reflected ultrasonic echoes from the second wave a second set of electrical signals; modifying the second set of electrical signals with phase correcting coefficients; and, comparing said first and second sets of electrical signals to provide a resultant signal indicative of the reflector at the focal line.
 2. A method according to claim 1 including providing each second wave generated with a predetermined phase shift pattern over the entire cross-section, and providing said phase correcting coefficients with values identical to that applied during the generating step but having a reversed sign.
 3. A method according to claim 1 including storing the derived first set of signals until said second set of signals are derived.
 4. A method according to claim 1 where the phase of each successive generated ultrasonic second wave is uniformly proportioned at each point defined by angle phi of their respective cross-sections.
 5. A method according to claim 4 whereby the phase is k phi , k being an integer.
 6. A method according to claim 5 whereby k
 2. 7. A method according to claim 4 whereby phi assumes a number of discrete values so that the phase of each in the ultrasonic second waves varies in steps over the cross-section.
 8. A method according to claim 7 whereby integral multiples of the discrete values of phi are equal to 2 pi .
 9. A method according to claim 7 whereby the stepped change is obtained by phase shift in each the generated ultrasonic second waves.
 10. Ultrasonic echo sounding apparatus for reducing echoes from reflectors outside a focal line with ultrasonic waves having substantially identical travel times comprising: annular assembly means having a plurality of identically sized segmental ultrasonic transducer means whose radiation surfaces are inclined towards an axis defined by the annular assembly means; generator means for driving said transducers; phase shift means adapted for connection between said generator means and transducer means; receiver means including a first and second adding means; said second adding means including phase rotation delay means; switch means operative in a first position for connecting said transducer means with said generator means and said first adding means, and in a second position for connecting said transducer means with said generator means through said phase shift means and said second adding means; and, means for comparing the output of said delay means and second adding means.
 11. Apparatus according to claim 10 wherein the phase shift means comprises a delay line which is tapped at regular intervals for connection to preselected ones of said transducer means.
 12. Apparatus according to claim 10 including: rectifier means connected from each of said adding means; and indicating means connected from said comparison means.
 13. Apparatus according to claim 10 whereby said ultrasonic transducer means includes twelve transducer elements commonly connected in groups of two through the switch means for being driven and delivering signals to the receiver means.
 14. Apparatus according to claim 13 whereby the outputs of the phase shift means have a relative phase difference of 60* and the phase rotation means apply the same phase shift with reVersed sign.
 15. Apparatus according to claim 11 whereby the rectifier means includes first and second multiplier means for respectively multiplying their output signals by cos omega t and sin omega t; first and second low pass filters respectively connected from said multiplier means for eliminating the frequencies close to the transmission frequency; and, circuit means for forming the root from the sum of the squares of their imputs.
 16. Ultrasonic echo sounding apparatus for reducing echoes from reflectors outside a focal line with ultrasonic waves having substantially identical travel times comprising: transmitter means; first, second and third annular ultrasonic transducer with annular radiation surfaces means having a common annulus axis; said first transducer means having a radiation surface inclined relative to said axis with edges disposed substantially in a plane perpendicular to the axis; said second transducer means having a radiation surface with edges disposed on a helical surface forming a raised portion with respect to said plane; said third transducer means having a radiation surface with edges disposed on a helical surface representing an indentation with respect to said plane; signal processing means; delay line means; subtraction means; indicating means connected from the subtraction means; and, switch means connecting in one position the first transducer means with said transmitter means and the signal processing means to the delay line means and thence to the subtraction means, and connecting in a second position the second transducer means to the transmitter means and the third transducer means directly to the subtraction means via said processing means. 